1. Field of Invention
The present invention relates generally to the production of bent glass sheets and, more particularly, to shaping hot glass sheets whose leading edge, as it is conveyed into the shaping station, is wider than the trailing edge.
2a. Technical Considerations
Bent glass sheets are commonly used as glaze enclosures in vehicles such as automobiles and the like. For such applications, the glass sheets must be bent to precisely defined curvatures dictated by the configuration and outline of the vehicle openings in which the sheets are to be installed, as well as the overall styling of the vehicle. At the same time, it is important that the bent sheets meet very stringent optical requirements so that the viewing area of the resulting shaped window is free of optical defects or marks that would interfere with good vision through the window. For certain windows used as glazed enclosures, bent glass sheets are tempered to strengthen them and increase their resistance to damage resulting from impact.
Often times, based on the desires of automobile stylists, the window curvatures are complex in shape. These windows require reverse S-curves along their leading edge and deep bends in their transverse direction. The complex curvatures generally necessitate the use of a press bending apparatus rather than sag bending apparatus to effect the final desired curvature. In press bending, the glass sheets are generally supported, either on a series of rolls or by a gas hearth bed, which supports the glass in close relation thereover by hot gas. The glass is then lifted from proximity to the support plane into engagement with a vacuum holder and then deposited onto a ring-like member which is interposed between the plane of support and the bottom surface of the vacuum holder.
The complicated nature of the heating operation required to soften the glass sufficiently to be shaped by press bending to the complex curvatures, causes the leading edge of the glass to develop a higher temperature than the trailing edge of the glass. This further complicates the shaping of the glass to various complex shapes because the leading edge of the glass which develops a higher temperature during conveyance to the furnace is more likely to sag than the trailing edge of the glass which develops a lower elevated temperature. Attempts to develop a reverse curvature transverse to the general longitudinal curvature of the sheets in a portion of the sheet near the furnace exit, i.e. the trailing edge, would result in overheating and additional sag of the leading edge, as well as inefficient heating.
Still another complication arises from the fact that the windows are often trapezoidal in shape with the leading edge and/or mid portion of the glass sheet being wider than the trailing edge. Because of the higher temperature of the leading edge required to make the complicated bends and its tendency to sag and deform as soon as it is unsupported, the glass sheet must be supported along its entire width as it is transferred from the heating furnace to its position on the gas hearth in the shaping station. This supported transfer must occur despite the fact that the gas hearth bed supporting the glass when positioned at the shaping station need only be trapezoidal in shape with its upstream support width being narrower than its downstream support width. If the corners of the leading edge and the longitudinal end portions of the glass mid portions are not supported, they will sag and contact the edge of the gas hearth bed as the hot sheet is initially transferred thereon.
It would be advantageous to fully support the entire glass sheet surface of glass sheets that are wider at their leading edge or mid portion than at their trailing edge and require complicated curvatures and specialized heat patterns as the sheets are moved into position in a shaping station, so as to minimize marring of the glass.
2b. Patents of Interest
U.S. Pat. No. 3,468,645 to McMaster et al. and U.S. Pat. No. 3,607,200 to McMaster both teach the use of a recessed lifting frame disposed below the surface of a gas hearth bed in the shaping station. The frame engages the periphery of a float glass sheet and lifts it off a flat gas hearth bed and into engagement with the shaping surface of an upper shaping mold so that the glass is curved in conformity with the upper mold. After shaping, the frame lowers the shaped sheet away from the mold and moves it into a cooling area. The frame in U.S. Pat. No. 3,607,200 includes a second portion that simultaneously removes a shaped glass sheet from the cooling section as the next shaped glass sheet enters. In both patents, the shape of the glass sheets and the final designed curvatures are relatively simple.
U.S. Pat. No. 3,846,104 to Seymour teaches the shaping of glass sheets to a non-uniform shape by positioning heat softened glass sheets over a gas hearth bed at a shaping station beneath an upper vacuum mold. The hot glass sheet exits the furnace and is transferred from the furnace gas hearth to the shaping station gas hearth without any intermediate support. A lower outline pressing mold of complimentary shape surrounds the gas hearth bed and is located below the upper vacuum mold. The lower outline pressing mold moves between a retracted position below the upper surface of the gas hearth bed and an upper position near the upper vacuum mold. The lower outline pressing mold raises to engage the glass sheet against the upper vacuum mold to cause the heat softened glass to develop a shape conforming to that of the vacuum mold. Suction pulled through the upper vacuum mold holds the shaped glass sheet against its lower shaping surface while the lower outline pressing mold retracts to enable a ring-like member having an outline shaping surface that conforms to the supported portion of the glass sheet adjacent its parimeter to shuttle into position below the vacuum mold. The vacuum pulled through the vacuum mold is ended and the shaped glass sheet falls onto the ring-like member for transfer to a cooling station where the glass is cooled sufficiently rapidly to develop a desired degree of temper.
U.S. Pat. No. 3,869,271 to Shaffer et al. teaches the mechanical support of a hot glass sheet as it is transferred from a first gas hearth bed in a furnace section to a second gas hearth bed in a shaping station. A series of snub rolls are positioned between the gas hearth beds along the conveyed path of the longitudinal end portions of the hot glass sheets. The upper surfaces of the rolls are aligned with the upper surface of the gas hearth beds so as to support the longitudinal end portions of the glass sheet as the sheet is conveyed from the first bed to the second bed.
U.S. Pat. No. 4,229,200 to Seymour teaches a drop forming arrangement wherein flat glass sheets are positioned on a flat gas hearth bed and lifted by means of a flat vacuum platen that contacts the upper surface of the glass sheet. A shaping ring is conveyed beneath the elevated glass and the vacuum in the platen is released allowing the glass to fall on the ring. Auxiliary shaping means are positioned around and below the upper surface of the gas hearth bed. After the sheet is lifted, the auxiliary shaping means extend above the gas hearth bed and prebend the sheet prior to dropping it on the shaping ring. If required, the auxiliary shaping means can be used to lift the glass sheet into engagement with the upper vacuum platen.
U.S. Pat. No. 4,432,782 to Seymour teaches the support of a flat glass sheet as it exits a furnace section by combination of a gas hearth bed and replaceable flat plate portions. The plate portions are slotted for movement of a lifting ring that is initially positioned below the plate and thereafter moves therethrough. As the hot glass sheet exits the furnace, the main portion of the sheet is supported by the gas hearth bed while the longitudinal end portions slide over the replaceable flat plate portions. To prevent marring of the longitudinal end portions of the hot glass that sags onto and contacts the plate portions, the plate portions are covered with a material such as boron nitride.